Performance comparison of four pre-filters used to prevent pitch category II PIO

Category II PIO cases are mostly characterized by actuator saturation. FWB, DS, DASA, RLF are four nonlinear pre-filters that have already been used to compensate for the phase lag led by actuator saturation, thus to prevent pilot-induced oscillation (PIO). Different means were used to make comparison among four pre-filters. Dscribing function was used to analyze and compare the phase compensation in open loop analysis by linearizing the nonlinear components of the fourpre-filters. The study mainly focused on pitch attitude PIO, in a pilot-vehicle closed loop system with a neutrally stable pitch bare aircraft that could easily cause PIO, three typical pitch flight simulation tasks were used to make comparison in terms of actuator velocity saturation prevention, pitch output dynamic characters, degree of phase compensation and alleviation of pilot operation burden, measured by pilot input magnitude. DASA turns out to be a better design among the four pre-filters. The result can provide a useful reference for filter design and pitch attitude PIO prevention study.